Underground combustion method for producing heavy oil



T. V. MOORE July 6, 1965 UNDERGROUND COMBUSTION METHOD FOR PRODUCING HEAVY OIL Filed Nov. 29. 1961 THOMAS v. MOORE INVENTOR.

y 5 7 c, 9411 AGENT United States Patent (3 3,193,008 UNDERGROUND COMBUSTION METHOD FOR PRODUCING HEAVY 01L Thomas V. Moore, Neuilly, France, assignor, by mesne assignments, to Esso Production Research Company,

Houston, Tex., a corporation of Delaware Filed Nov. 29, 1961, Ser. No. 155,771 3 Claims. (Cl. 166-11) This invention is directed to the recovery of high viscosity petroleum deposits from subterranean reservoirs. Broadly, the invention is concerned with the recovery of viscous petroleum by in-situ combustion from a reservoir characterized by a gas permeability which is too low to permit successful application of conventional combustion techniques. More specifically, a method is provided which involves the propagation of a combustion front Within such a reservoir at the oil-water transition zone.

"It is known to recover relatively immobile petroleum deposits by underground combustion utilizing an existing well communicating with the oil reservoir as an input well and employing one or more existing wells laterally spaced therefrom as the producing Well. The method involves the initiation of combustion along the bore of the input well and a continuous injection of air or oxygen into the formation with the result that a burning wave or front gradually advances toward the output well or wells. In order to sustain and advance the burning front, a continuous stream of gases must pass from the input well through the front, and the products of combustion must pass from the combustion front to the output well.

In some areas, however, the diihculty in establishing gas permeability throughout the horizon between the input and output wells is so great as to prevent a successful application of the combustion process. In many such reservoirs, the impermeable zone containing viscous oil is underlain by a water zone in which it is comparatively easy to establish gas permeability. Intermediate the oil zone and the water zone there lies a transition zone having an oil content progressively higher in the direction of the oil zone and a water content progressively higher moving into the direction of the water zone.

In accordance with the present invention, an injection well is completed in such a reservoir at a depth within the transition zone just sufficient to establish gas permeability. The output well is completed at approximately the same depth. At this level within the transition zone the oil content is insufli-cient to block gas permeability and yet is sufficient to sustain a combustion front. In extreme cases where the depth required to establish gas permeability is so near the water zone that it becomes difficult to sustain the combustion wave with air, it is nevertheless practical to establish the combustion wave using pure oxygen or oxygen-enriched air at this level.

A major problem normally encountered during underground combustion operations is the characteristic overburn caused by gravity segregation which restricts combustion to the upper layers of the oil-bearing sand. This results in only a fraction of the oil sand being burned free of hydrocarbons and, moreover, causes a direct loss to the overburden of a large part of the heat generated.

In accordance with the present invention, overbur-n within the oil-water transition zone becomes a great advantage of the process. The inherent tendency of the combustion front to move upward accelerates the transfer of heat and miscible gases into the region of increased oil saturation, thereby reducing the viscosity of the oil. As its viscosity is lowered, the oil migrates to the production well or wells, where it is then lifted to the surface in a conventional manner.

A more detailed description of the invention is provided by reference to the accompanying drawing which shows a 3,193,008 Patented July 6, 1965 cross-section of a viscous reservoir 11 having oil-water transit-ion zone 12 and water zone 13. The reservoir is penetrated by injection well v14 and production- Well 15 having perforations 16, 17, 18 and 19. Instead of the cased and perforated completion shown, each of the wells may in some cases be completed with an open-hole bottom.

Once it is determined that the oil in zone 11 is too viscous to produce through natural flow an attempt is made to establish in-situ combustion by injection of air, oxygen, or oxygen-enriched air directly into the oil zone. However, in the reservoir shown it has been determined that permeability to gas is too d-ifiicult or impossible in the oil zone to establish and maintain a combustion front. Injection well 14 is then completed progressively farther into the reservoir and into the oil-Water transition zone. Repeated attempts to establish gas permeability at increasing depths in the oil-water transition zone must eventually be successful since it is known that a water reservoir is gas permeable. The applicability of the invention is not determined by the depth necessary to establish gas permeability since combustion can be sustained even though permeability is first established at a level well into the transition zone.

Once permeability to gas is established combustion is initiated at the injection well bore by conventional means such as down-hole electric heaters. Where permeability to gas is established only in the lower level of the transition zone, the concentration of oil will sometimes be too low to establish and maintain a combustion front using air alone. However, in such circumstances oxygen-enriched air or pure oxygen is readily available for use in establishing the combustion front. Moreover, when it becomes necessary to resort to use of oxygen-enriched air, it is no longer essential that combustion be established in the immediate vicinity of the injection well. That is, the enriched oxygen is capable of initiating spontaneous combustion once a sutlicient concentration of oil is encountered. Even in the extreme case of oxygen injection into the water zone, combustion will eventually result since the efiect of gravity will force the oxygen to migrate upward and eventually contact suificient amounts of oil to cause spontaneous ignition. The injection of oxygenenriched air or oxygen is then continued with the result that combustion spreads through the transition zone, generating burned-out area 20.

During the earlier stages of combustion, little or no flow from the production well is experienced. However, the heat released from the combustion in the oil-water transition zone rises into the viscous oil zone 11 with the result that a portion of the viscous oil is reduced in viscosity. Combustion gases and cracked gases are also forced into the viscous oil zone thus further contributing to the increased mobility thereof with the ultimate result that oil is produced from well 15.

Eventually, a breakthrough of the combustion front occurs at the production well. If the breakthrough is premature and causes the injected air to by-pass unburned regions, the deepest completions in the injection well may be sealed off and air then forced into a higher level of the reservoir than before. This is now possible, due to reductions in the oil viscosity, whereas the reservoir was initially impermeable at the higher levels.

Similarly, the lowermost completions in the production well or wells are now sealed off, and production is established at a higher level. Just as at the injection well, the viscosity of oil has been reduced sufiiciently to permit gas permeability and oil flow at a higher level than at the beginning of the process.

Combustion is then reinitiated and sustained at the higher level within the reservoir until breakthrough again forces interruption. Completions are again sealed off oxygen or oxygen-enriched air. a

As an alternative embodiment, preferable in certain reservoirs, the input Well completions are sealed off at I the level of initial injection soon after permeability to gas can be established at a new level substantially thereabove. The injection level is moved up-hole in such instances before the occurrence of oxygen breakthrough at the production well. Thus, it is frequently practical or impermeable to gas, (2)

even desirable to continue production at the same level as before, moving up-hole in the input well only. At any rate, the considerations which determine the preferred level of production from time to time are not the same as determine the level of injection; Accordingly, ataj given time the production level may be either below, above, or the same as the injection level. Ultimately, as before, it is contemplated that both the input'and the output well completions will be moved to progressively higher levels within the reservoir until substantial deple tion thereof is accomplished.

Past experience with underground combustion has re vealed that only a relatively small amount of oil saturation is'required to sustain the. combustionfront. In many instances, as little as 250 barrels of oil per acre-foot of sand is consumed. This represents about 10 to of the pore volume in a formation which is -25% :porous. Thus it .becomes clear that a combustion wave is readily sustained in the] oil-water transition zone, even where the water-to-oil ratio is initially high. The water presents no interference with combustion, since most of itis displaced ahead of the combustion front by the gaseous products of combustion, and the residual nitrogen or other inert gases associated with the injected oxygen.

The oxygen containing gas is'furnished to the formation at a rate which is initially low and which is increased with timein' an approximately linear relationship. When air is used, about 200 to about 1,000 cubic feet of air per hour, for example about 500 cubic feet per hour, is supplied to the burning zone per square foot of combustion front. Somewhat lower rates are suitable when injecting In additionto the forward combustion technique as just described, various other modifications of the under-. i

ground burning process are known inthe art, and are also suitable for use in accordance with the instantine I vention.

For example, in the reverse combustioni'process the cornbustion front is initiated at a first well, and 'is then sustained by the injection of an oxygen-containing gas via a second Well laterally spaced therefrom T hus, the front advances toward the injection well or wells, and

nated. a

, Still further, combinations of forward and reverse burning are also'useful in carrying out'the present invention.

oil is produced fromthe well at which the front origi-.

While various embodiments of the invention have been specifically described, it is obvious that further modifications will occur-to those skilled in the art. Accordingly,

- it is intended to include all such modifications Within the scope of the following claims; 7

What is claimed is f 1. A method forthe recovery of highly viscous 'petro- ,said input. well in said transition zone at a depth sufficient to establish therein permeability to gas,'injecting an oxygen-containing gas via: said input well into said 7 transition zone only, under combustion-sustaining conditions, whereby said'viscous petroleum is mobilized,'and

producing said petroleum from said outputwell.

2. A method for the recovery of highly viscous petroleum from anundergroundreservoir penetrated by an input well and an output well, said reservoir having an oil zone Which is substantially impermeable to gas, having an oil-water transition zonebelow said petroleum, and a water zone below said transition zone, which comprises completing said input well in said transition zone at a depth sufficient to establish therein permeability to gas, injecting an oxygen-containinggas via said input well into said transition zone under cornbustionvsustaining conditions whereby said viscous petroleum is mobilized, then producing petroleumirom said output Well, interrupting said injection when: permeability togas can be "established at a new level substantially above the level ofinitial injection, scaling off said injection .Well at the level of initial injection, and resuming the injection of an oxygen-containing gas atcombustion-sustaining conditions at said new level while continuing to produce petroleum from said output well.

3. A method for the recovery of highly viscous petroleum' from'an underground reservoir penetrated by an input ,well and an output well, said reservoir having an oil zone which is' substantially impermeable to gas, having an oil-water transition zone below said petroleum and a through, sealing. off said injection well at the level of initial injection; then resuming the injection of an oxygen containing gas at combustion-sustaining conditions at a new level within said input wellsubstantially abovefthe level of initial injection, and resuming the production of petroleum from said output well ata level substantially above the level of initial production. I

v ReferencesCited by the Examiner V t 1 UNITED STATES rA'rENTs 2,874,777 l OTHER. REFERENCES 7 i Musliat, Morris, Physical Principlesof Oil Production,

first edition, 'McGraw-Hill' Book Company, Inc., New York (l9 49),'page l05.f

BENJAMIN HERSH, Primary Examineri CHARLES E. OCONNELL'Ea-aminer. 

1. A METHOD FOR THE RECOVERY OF HIGHLY VISCOUS PETROLEUM FROM AN UNDERGROUND RESERVOIR PENETRATED BY AN INPUT WELL AND AN OUTPUT WELL, SAID RESERVOIR BEING CHARACTERIZED BY (1) HAVING AN OIL ZONE WHICH IS SUBSTANTIALLY IMPERMEABLE TO GAS, (2) HAVING AN OIL WATER TRANSITION ZONE BELOW SAID PETROLEUM ZONE, AND (3) A WATER ZONE BELOW SAID TRANSITION ZONE, WHICH COMPRISE COMPLETING SAID INPUT WELL IN SAID TRANSITION ZONE AT A DEPTH SUFFICIENT TO ESTABLISH THEREIN PERMEABLITITY TO GAS, INJECTING AN OXYGEN-CONTAINING GAS VIA SAID INPUT WELL INTO SAID TRANSITION ZONE ONLY, UNDER COMBUSTION-SUSTAINING CONDITIONS, WHEREBY SAID VISCOUS PETROLEUM IS MOBILIZED, AND PRODUCING SAID PETROLEUM FROM SAID OUTPUT WELL. 